With advance of the size shrinking technology in recent years, power consumption of LSIs becomes greater and power consumption of semiconductor memories at the time of standby becomes normegligible. The power gating technique is used to turn off the power supply for an unused part of an LSI and turn on the power supply only for a part in use. In current semiconductor technology, a volatile SRAM is used as a memory circuit and consequently the power supply cannot be turned off for the memory circuit. In a memory circuit using an SRAM or a DRAM which is volatile, sudden turnoff of the power supply causes loss of memory information.
As a nonvolatile memory circuit based on the current semiconductor technology, a nonvolatile memory of resistance change type is proposed. In the nonvolatile memory of resistance change type, information is stored in the SRAM which is a semiconductor memory when its power supply is turned on. Even if the power supply is turned off, information remains stored in the nonvolatile memory. When not performing a read or write operation, therefore, the power supply can be turned off. If a nonvolatile memory is used in an LSI, therefore, power gating can be performed easily. Accordingly, the nonvolatile memory of resistance change type is attracting attention.
Research and development of spin electronic devices utilizing the degree of spin freedom of electrons have been performed vigorously in recent years. Research and development based on a tunnel magnetoresistive effect (TMR) have been performed vigorously and applied to a magnetic random access memory (MRAM) and a hard disc drive (HDD). In addition, the spin transistor obtained by coupling semiconductor with a magnetic material is attracting attention.
The magnetic material can have a nonvolatile memory function. If the magnetic material is used as a nonvolatile memory, its application to the power gating technique and memory backup can be anticipated.
When fabricating a memory circuit by using the CMOS technique of the semiconductor, a volatile SRAM is used as a memory which stores information. Since the SRAM is volatile, the power supply cannot be turned off even at the time of standby, resulting in high power consumption. Furthermore, since the SRAM is volatile, memory information is lost at the time of sudden turnoff of the power supply. In the case where large quantities of SRAMs are used, power consumption due to a leak current becomes greater even when they are not operating. This results in a circuit which is hard to be highly integrated.
In addition, since the SRAM is a volatile memory which loses information if the power supply is turned off, it is necessary to write information stored in an external memory into the SRAM whenever the power supply is turned on. This results in a problem that it takes trouble and time when the power supply is turned on. Furthermore, when the SRAM is used, there is a problem that it is necessary to secure an external memory for storing information when the power supply is turned off, and power consumption and a capacity are needed for the external memory. Therefore, this becomes one of the causes which hinder higher integration and lower power consumption in the whole system.
Since the SRAM uses six devices for a 1-bit memory, there is a problem that it requires a larger area.
Furthermore, since the operation speed of the LSI is largely influenced by an interconnection delay, use of an SRAM having a large area as a memory makes the operation speed of the circuit low.